More than two million adults in England could be eligible for weight loss surgery, researchers say.

Operations for weight loss - also called bariatric surgery - such as the fitting of a gastric band or gastric bypass surgery, is currently available on the NHS to treat those with potentially life-threatening obesity.

Some 8,000 people receive the treatment each year, but researchers from Imperial College London say more than 5% of adults would qualify, more than 2.1 million people.

Their study, published in the Journal of the Royal Society of Medicine, found that despite evidence of the cost-effectiveness of weight loss surgery, surgery rates were estimated to be a third of the NHS's benchmark rate.

The number of people eligible for surgery "far exceeds" the estimated number of people having procedures.

Researchers, who examined the health of nearly 9,500 people, acknowledged the huge financial implications for the NHS if treatments were carried out in such greater numbers.

But they said more investment may be necessary to meet a growing need for the procedures.

According to NHS guidelines, weight loss surgery may be offered to those whose morbid obesity could kill them, or to people who are morbidly obese and who have a serious condition such as type 2 diabetes or high blood pressure that could be improved if they lost weight.

The study found that those meeting the criteria for bariatric surgery were more likely to be women, retired with lower educational qualifications and of lower socio-economic status.

It said: "Recent hospital episode statistic data show surgery rates from 2003/4 to 2009/10 have risen year on year in England; with rates highest in women and those aged 40-54 years.

"However, service delivery rates still fall significantly below the level needed to support all those who could potentially benefit."

Several factors contributed to surgery rates being so low, the study found.

Those with greatest need for surgery were more often in the lowest socio-economic groups, who are least likely to use healthcare services.

Patients' awareness of the possibility of surgery and their commitment to make the necessary lifestyle changes prior to the treatments may also be barriers, researchers said.

Doctors might not be correctly identifying or referring those eligible, and service provision may also be insufficient.

The study concluded: "5.4% of the general adult population is eligible for bariatric surgery in England, far exceeding the current bariatric surgery uptake.

"Due to the limited capacity of health services to meet demand under existing criteria, greater investment into service provision may be required to meet a growing need.

"This would have significant resource implications. Since those eligible are more likely to be of a lower social class and have lower qualifications, such resources would need careful allocation to ensure equitable access on the basis of need."

Lead researcher Dr Sonia Saxena said: "Despite clear guidelines outlining who can undergo such surgery with the NHS, and evidence that these procedures are cost-effective in the long run, less than 1% of those eligible have weight loss surgery each year," the BBC reported.

"This raises questions about why more procedures are not currently being carried out."